In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plates, also called printing plate precursors, typically comprise an imageable layer applied over the surface of a hydrophilic substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
If after exposure to radiation, the exposed regions of the coating are removed in the developing process, revealing the underlying hydrophilic surface of the support, the plate is called as a positive-working printing plate. Conversely, if the unexposed regions are removed by the developing process and the exposed regions remain, the plate is called a negative-working plate. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution, and repel ink.
Direct digital imaging of offset printing plates, which obviates the need for exposure through a negative, is becoming increasingly important in the printing industry. Thermally imageable, multi-layer imageable elements that comprise a hydrophilic substrate, an alkali developer soluble underlayer, and a thermally imageable layer have been disclosed. On infrared exposure, the exposed regions of the imageable layer become soluble in or permeable by the alkaline developer. The developer penetrates the imageable layer and removes the underlayer, revealing the underlying substrate.
The imageable layer of these elements comprises a dissolution inhibitor, which suppresses removal of the imageable layer in the unexposed regions. Materials that comprise the o-diazonaphthoquinone group are excellent dissolution inhibitors. However, because o-diazonaphthoquinones are sensitive to “white light” or “room light,” the resulting imageable element is sensitive to white light. Thus, a need exists for a method of decreasing the white light sensitivity of o-diazonaphthoquinone containing positive-working thermally imageable elements.